The invention is based on a state of the art, as it is known from DE No. 31 23 502 C2, which corresponds to U.S. Pat. No. 4,477,214. The machine described in this patent is intended in the first place for the precision working of the tooth flanks of not (yet) hardened gears, thus for a shaving of the gear with a shaving gear, namely, according to the parallel, diagonal and underpass methods. However, the machine can also be utilized in a limited manner for the precision working of the tooth flanks of hardened gears, thus for the honing, fine finishing, finish grinding, etc. (a uniform definition of these methods does not exist up to now) with an abrasive tool. In particular during the diagonal and underpass methods, the tool is worn rather quickly, so that the methods, in spite of the shorter machining time compared with the parallel method, are so far not widely used.
If the gears to be machined are to receive crowned flanks, then it is possible with the parallel and diagonal method to create evenly crowned flanks only over their entire width, since the tool, in dependency of the longitudinal or diagonal feed, to which is to be added a radial infeed or plunge feed, carries out a swivel movement.
Also a method for the precision working of hardened gears is known ("Werkstatt und Betrieb", 118th year (1985), No. 8, Pages 505 to 509, here: Page 506), in which the abrasive tool is moved parallel to the workpiece axis and a radial infeed or plunge feed occurs intermittently thereto, with a swivel movement of the workpiece being superposed over the longitudinal feed for manufacturing crowned tooth flanks. The longitudinal feed, however, demands--just like in the case of the method mentioned in the beginning--long machining times and should therefore be avoided.
The tool is in another known method supplied exclusively radially, thereby permitting a shorter machining time. In order to machine the workpiece tooth system over its entire width, the tool tooth system must abut the workpiece tooth system, which creates certain problems during the tool manufacture. These problems are magnified when crowned tooth flanks are to be created, since the crowning of the workpiece must be worked additionally in a complementary form into the tool.
Tooth flanks, which are crowned only at their end areas, that is, in the areas adjacent to the axially facing tooth faces, can only be created in the underpass method with tools which are extremely difficult to manufacture.
Starting out from the disclosed disadvantages of the known methods, the basic purpose of the invention is to further develop the initially mentioned method so that crowned tooth flanks can be created on hardened gears in an acceptable amount of time with the least possible tool wear.
This purpose is attained with a method wherein the tool during at least one feed movement without plunge feed carries out a tilting movement about an axis which is at least approximately tangent to the rolling cylinder of the tool and is directed both perpendicularly with respect to the common normal and also perpendicularly with respect to the workpiece axis. It has proven to be sensible for the machine adjustment or rather programming to first, very normally, precision work the workpiece teeth and to remove only at the end of the machining task the requisite amount of material for crowning the teeth at the tooth ends.
A significant improvement is achieved with the method wherein:
(a) The feed movements, which occur in connection with the plunge feed, are dimensioned such that the path covered by the crossed-axes point on the workpiece in axial direction, ends before the crossed-axes point has reached the edges of the tooth flanks at the axially facing sides of the tooth,
(b) the feed movements, which occur without plunge feed, are dimensioned such that the path covered by the crossed-axes point on the workpiece in axial direction, ends the earliest when the crossed-axes point has reached the tooth flanks, and
(c) the tool carries out only during at least one feed movement without plunge feed a tilting movement about an axis which at least approximately is tangent to the rolling cylinder of the tool and is directed both perpendicularly with respect to the common normal and also perpendicularly with respect to the workpiece axis, which can be supplemented by all feed movements occurring with the plunge feed being equally long.
It has been discovered that the mentioned increased tool wear is caused by the hardness at the ends of the workpiece teeth. Thus, material removal is to occur so to speak "from the inside", namely the machining occurs first primarily in the slightly less hard area of the tooth flanks without crowning being considered, and only toward the end of the precision working is the material removed at the very hard tooth ends and the desired crowning created at the same time. The machine according to the aforesaid DE No. 31 23 502 C2 and U.S. Pat. No. 4,477,214 is not suited to carry out the inventive method. A suitable machine will be described later on.
The first machined slightly less hard area can, as long as an infeed movement (plunge feed) still occurs, remain the same, however, it can also little by little be increased. However, the crossed-axes point should never wander beyond the edges of the tooth flanks at the axially facing sides of the workpiece.
Because of the danger of a secondary burr formation at the pointed tooth ends in helical tooth systems, one lets the feed movements advantageously start off-center, namely, at a point adjacent to the blunt tooth ends, which can simultaneously be a reversal point of the feed movements. The speed of the tool-workpiece pair should also be increased at this point at the start of the precision working.
The inventive method can be further developed wherein the plunge feed occurs initially at a first feed speed or step size and, after reaching a selectable center distance between the tool and the workpiece, there occurs a second feed speed or step size, which is reduced compared with the first one, or wherein the plunge feed occurs initially at a first feed speed or rather step size and is then regulated down to at least a second feed speed or step size, which is reduced compared with the first one, through a measuring means for sensing forces and/or deflections and/or torques and/or noises and/or other vibrations, measured by said measuring means, which is very advantageous for the quality of the surfaces produced on the workpiece flanks.
In the case of the method which is being discussed here, the tool is always moved relative to the workpiece in one direction, which extends inclined with respect to the workpiece axis. Just like in gear shaving according to the diagonal method, machining could be done with a short feed path if the tool tooth system is correspondingly corrected, namely, is of a crowned design, in order to machine the tooth system of the workpiece over its entire width. Because of the inventive measure to permit the tool to carry out a tilting movement at the end of the precision working in order to produce a crowning on the tooth flanks, it is, however, possible to use a tool which does not have corrected tooth flanks.
Depending on the width of the tool and the angle, chosen for the feed direction with respect to the workpiece axis, already a relatively short feed path is sufficient in particular in the method mentioned above or in the feed movements without plunge feed also mentioned above to shift the point of contact or rather the common normal from one axial side of the workpiece tooth system over the tooth width to the other axial side. It is therefore advantageous to further develop the method wherein the relationship ##EQU1## exists between the diagonal angle .epsilon. and the crossed-axes angle .phi. in order to obtain a feed path which is as optimal as possible.